Hi,
There is no option to do it.
However, you should be able to do it with a bit of coding and a mass action.
For example, you can create a mass action "order" with a trigger "after an order is updated" and with an action "run PHP code" with such code:
$db = JFactory::getDBO();
$db->setQuery('SELECT count(order_id) FROM #__hikashop_order WHERE order_user_id={order_user_id} AND order_status IN ("confirmed", "shipped")');
$number_of_orders = $db->loadResult();
if($number_of_orders >= 3) {
$userClass = hikashop_get('class.user');
$hkUser = $userClass->get({order_user_id});
if(!empty($hkUser->user_cms_id)) {
$user = clone(JFactory::getUser($hkUser->user_cms_id));
jimport('joomla.access.access');
$userGroups = JAccess::getGroupsByUser($hkUser->user_cms_id, true);
$userGroups[] = XXX;
$user->set('groups',$userGroups);
$user->save();
}
}
where XXX is the id of the user group you want to add to the user account.